Special counsel Robert Mueller and his prosecutors have invoked an unusual "conspiracy to defraud the government" charge to ensnare a Russian cyber network and could use the same legal strategy to go after President Trump and his associates, even if the conspiracy is not linked to a criminal act.

Although the president constantly denies any wrongdoing, Mr. Mueller's 10-month-long investigation has become the heart of the Trump-Russian election meddling saga, dominating headlines and leaving the White House tangled in conflict and controversy as additional charges have also been filed against four of his close associates - with further indictments likely.

Last month, Mr. Mueller, a Republican and former FBI director, indicted 13 Russian nationals connected to the Internet Research Agency (IRA) Russian "troll farm," accusing the IRA of interfering in the 2016 U.S. presidential election by spreading fake news stories through U.S. social media. The same approach was employed in securing a plea deal last month with Rick Gates, the aide for former Trump campaign chairman Paul Manafort.

What has attracted attention in legal circles is the underlying legal theory behind the indictments, accusing the Russians of essentially committing a crime by preventing agencies of the U.S. government from carrying out the duties. Mr. Mueller appears to be leveraging the theory "into a powerful instrument with respect to both foreign and domestic actors," according to a recent article on Lawfare, a national security blog by the Lawfare Institute and Brookings Institution.

Emma Kohse, Harvard International Law Journal editor-in-chief, Benjamin Wittes, a senior fellow at the Brookings Institution and the Lawfare blog's top editor, argue that, based in the language of the indictments and the legal precedents behind them, the "conspiracy to defraud the government" charge provides the Mueller team with significant flexibility in trying to build a case against Mr. Trump and members of his 2016 campaign.

Mr. Mueller's prosecutors charge in the indictment that the Russian defendants conspired to obstruct U.S. government agencies in multiple ways, including improper campaign expenditures, failing to register as foreign agents carrying out political activities and also deceitfully obtaining visas - in essence blocking the Federal Election Commission and the State Department, among other agencies, from doing their jobs.

In addition, several of the defendants "traveled to the United States under false pretenses for the purpose of collecting intelligence" to assist the troll farm's influence operations.

The same legal theory appears to underpin at least part the Mueller team's case against Mr. Gates. In a February filing, the special counsel alleged that Mr. Gates, who agreed to a plea deal admitting financial fraud and lying to investigators, defrauded the government by "impeding, impairing, obstructing, and defeating the lawful governmental functions of a government agency, namely the Department of Justice and the Department of the Treasury."

The crime of "conspiracy to defraud the United States," the authors noted, is not new, but sets up a very different legal battle than trying to prove outright that Mr. Trump or his associates actively colluded with Russians to alter the outcome of the 2016 election.

The "conspiracy to defraud" approach, the Lawfare authors point out, "has been sitting in plain sight in the general conspiracy statute ... since 1948 (and an earlier provision with substantially similar language dates to 1867). The statute makes it illegal for two or more persons to 'conspire either to commit any offense against the United States, or to defraud the United States, or any agency thereof in any manner or for any purpose.'"

The authors add that "unlike conspiracy to commit an offense, conspiracy to defraud the United States need not be connected to a specific underlying crime," and "defraud" is not defined by the law.

Mr. Mueller's IRA indictment uses the statute specifically in context of U.S. election law and while its use is not popular, several federal circuit courts have heard cases based on the theory and allowed its application regarding federal election laws.

"Russia's sustained, many-front effort to interfere in the 2016 election - a glimpse of which was shown in the indictment - is undoubtedly more complex and far-reaching than the plots that have previously been prosecuted as conspiracies to defraud the United States," Ms. Kohse and Mr. Wittes write. "But that makes Mueller's theory richer, not more novel."

Because Mr. Mueller is alleging conspiracy to defraud the FEC, DOJ and State Department, some legal scholars say recent commentary about "collusion" between the Trump campaign and Kremlin is misdirected. Instead, he appears to be building a case with the FEC, DOJ and State Department at its core - not the Kremlin.

And, the authors note, it's not much of a legal stretch to go after U.S. citizens who aided the Russian troll farm in the conspiracy against U.S. government agencies.

"To allege that a new conspirator had joined such a conspiracy, Mueller would have to allege only that such a person - presumably a new defendant - had agreed to participate in a scheme of deceit by which the FEC, the Justice Department or the State Department was deprived of its regulatory authority," Ms. Kohse and Mr. Wittes write.

Following that line of legal logic, any Americans who knowingly assisted the IRA troll farm - by feeding them information or directing them on how to employ that information - would be taking part in its scheme "to influence the U.S. election behind the back of the U.S. government." They would thus be guilty of committing a crime - even if what they though they were doing was legal.

They also could very likely have not known that they were assisting a Russian plot to meddle in the 2016 election, U.S. intelligence sources have repeatedly told The Washington Times.

"Mueller is targeting the 'useful idiots' who worked as Russian propagandists who didn't even realize who they were working for," a source told The Times.